Febrile children account for 10 to 20 percent of pediatric visits to emergency departments. In most of these children, the fever is caused by viral illness, but approximately 2 to 4 percent of febrile children under two years of age with no source of infection have bacteremia. It is often difficult for physicians to determine which children have viral infections and which have bacterial infections. Several clinical factors, including the height of the fever and the complete blood count (CBC), are useful diagnostic tools. Many physicians assume that the presence of band neutrophils in the blood indicates a bacterial illness. Kuppermann and Walton conducted a prospective study to determine if the percentage band count, the absolute band count (ABC) and the band-neutrophil ratio (BNR) in the peripheral blood smear differ between young febrile children who have documented viral infections and those who have bacterial infections.

Patients selected for the study were children under two years of age who were seen in the emergency departments of three pediatric hospitals. Patients had to have a temperature of 38°C (100.4°F) or higher if younger than three months of age or 39°C (102.2°F) or higher if three to 24 months of age. Children were excluded from the study if they had been immunized or had taken antibiotics in the previous 48 hours, had a clearly identifiable infection on physical examination, had a known chronic illness or were immunocompromised.

All of the children were evaluated and scored using the Yale Observation Scale (YOS) and a CBC, manual differential count and blood culture were obtained. Urine cultures were obtained from all girls and from boys who were under six months of age. All children who were younger than two months of age underwent a lumbar puncture. Nasopharyngeal specimens for the detection of adenovirus, influenza virus, parainfluenza virus and respiratory syncytial virus (RSV) were obtained from all children whose guardians gave consent. Finally, a chest radiograph was obtained for children with signs of a lower respiratory tract infection. Any child with a lobar infiltrate was excluded from the study because of the difficulty of differentiating viral from bacterial etiology.

Of the 432 children enrolled in the study, 31 had documented bacterial infections. Nasopharyngeal specimens were obtained from 226 of the children, and 75 of these children had a documented respiratory viral infection. Fifteen of the 432 children had lobar pneumonias and were subsequently excluded from the study. The 316 children who had no specific diagnosis were also excluded, leaving a total of 100 children for the primary analysis.

The children with bacterial infections were significantly older and had significantly higher temperatures, white blood cell (WBC) counts and absolute neutrophil counts than the children with viral respiratory illnesses. No significant differences in the YOS scores, the percentage band counts (9.6 vs. 10.3 percent), the ABC (1.5 vs. 1.7 percent) or the BNR (0.19 vs. 0.24 percent) were found between the bacterial and the viral groups, respectively. In a subanalysis that compared patients with bacteremia or a urinary tract infection (UTI) with patients who were culture-negative, the mean band counts were 13.5 percent in the first group and 10.9 percent in the second group. Another comparison included children with UTI and those with RSV. The mean band counts were 15.2 percent in the UTI group and 13.4 percent in the RSV group—numbers that added no significant predictive information after adjusting for age, temperature and the YOS score.

The authors conclude that in febrile children, the variables that are most often associated with a bacterial infection include age, height of temperature, WBC and absolute neutrophil count. The band count, whether viewed as an absolute number, a percentage or a ratio (e.g., BNR), does not help to distinguish a viral infection from a bacterial infection. The authors also state that if the physician obtains a CBC as part of the evaluation of a febrile child under two years of age, routine manual inspection of the peripheral blood smear to determine the band count is unnecessary.